Infection with hepatitis B virus is a major cause of liver disease worldwide and accounts for 5-10 per cent of chronic liver disease and cirrhosis in the United States. Safe and effective vaccines are available for prevention of hepatitis B, but therapies for the disease once it has occurred are limited in efficacy. Alpha interferon was the first antiviral agent shown to be effective in this disease and was licensed for this indication in 1993. The basis for licensure were, in part, studies conducted by the Liver Diseases Section, NIH. However, alpha interferon is effective in one-third or less of patients with typical chronic hepatitis B and its role in atypical forms remains unclear. Current activities in the Liver Diseases Section are focused on developing better therapies to prevent long-term consequences and analysis of immunological factors that predict or correlate with outcome of treatment. A longterm study of lamivudine (3-thiacytidine) for both HBeAg positive and anti-HBe positive chronic hepatitis B has been underway since 1996. A total of 42 patients have been enrolled into this study. Virtually all patients had a serum biochemical, virological and histological response to lamivudine in a dose of 100 mg once daily. However, breakthrough with viral resistance has occurred in 80% of patients who were initially HBeAg positive and 40% of those initially anti-HBe positive (and HBeAg negative). Liver biopsies taken after 3 to 5 years of therapy show marked improvements in patients who maintained a viral response, but little or no improvement in those with viral resistance. Most striking has been a resolution of fibrosis in patients with a long-term maintained response to therapy and, in some cases, return of the liver biopsy to normal. A central issue is how to manage patients who develop lamivudine resistance. Patients with resistance demonstrating progression of disease have been treated with alpha interferon which has induced transient improvements but without clearance of HBeAg. Two protocols have been developed to use the oral nucleotide analogue, adefovir for chronic hepatitis B: one for patients with lamivudine resistance (developed in collaboration with the Laboratory of Immune Regulation, NIAID) and one for previously untreated patients who will receive the combination of lamivudine and adefovir longterm with similar monitoring and follow up as done for patients who received lamivudine monotherapy. Studies in planning include analysis of T cell reactivity to HBV antigens and correlations of these to clinical responses to antiviral therapy. These studies have led to the proposal to use HBV vaccination to augment T cell responses in treated patients. Protocols incorporating use of vaccine immunotherapy are being developed. These studies will help define the efficacy and safety of long-term, continuous antiviral therapy of hepatitis B.